The present invention relates to heat engines and more particularly to heat engines having a dual working-fluid, parallel-compound thermodynamic cycle with improved thermal efficiency and simplicity.
It is well known that the Carnot cycle is a hypothetical thermodynamic cycle used as a standard of comparison for heat engine cycles such as the Brayton, Rankine, Otto and Diesel. This cycle shows theoretically that even under ideal conditions a heat engine cannot convert all the heat energy supplied to it into mechanical energy. Some of the energy must be rejected as waste or exhaust heat. The maximum efficiency of the Carnot cycle is given by: EQU Efficiency.sub.Carnot = 1 - T.sub.2 /T.sub. 1.
here, T.sub. 1 is the maximum engine cycle temperature and T.sub.2 is the temperature of the working fluid after extracting the engine work. To improve the cycle efficiency, one can increase T.sub.1 and/or lower T.sub.2. In other words, the greater the temperature ratio between the source and sink, the greater the engine efficiency.
Increasing T.sub.1 sooner or later is limited by the strength and durability of materials in high temperature environment. The lower limit of T.sub.2 under normal conditions is the ambient temperature. Current heat engines do not generally push T.sub.2 towards ambient temperature. Only the combined cycle engine, combining Brayton and Rankine cycles in series such that the Rankine cycle is operated by the temperature difference between the exhaust temperature most nearly accomplishes the objective of high temperature operation between source heat and sink temperature of the Brayton cycle and the ambient temperature. These types of cycles are summarized subsequently. Lower T.sub.2 temperature enables the use of high expansion pressure ratios, hence greater mechanical work extraction.
In conventional heat engines, the working fluid must be compressed to high pressures before heat energy is added to the working fluid. The heated working fluid is then expanded through an expander to convert the added heat energy into mechanical work. The net mechanical work output is the heat energy converted to mechanical work minus the mechanical work required to compress the working fluid, minus any heat losses in the system. It is to the advantage of an engine system to use as little compression work as possible.
This is possible in a liquid-vapor system since liquid is almost an incompressible fluid. To compress a liquid (for example, water, freon, or mercury) to 3,000 psia requires a small, practically negligible amount of energy in comparison with the energy required to compress a comparable mass of a gas, for example, air.
The Rankine thermodynamic cycle, typified by the conventional steam engine, takes advantage of phase change of a fluid between the liquid and vapor phases to minimize the compression work. However, this change of phase requires additional energy to overcome the latent heat of vaporization of the fluid. And, after expansion, a large amount of heat has to be absorbed from the working fluid just to convert the vapor back into a liquid. This is done in a closed loop cycle where the same working fluid is recycled continually through the cycle. The alternative is to simply exhaust the vaporized working fluid and not attempt to convert it back into a liquid. This is what is done in an open loop cycle. Hence, to optimize a working cycle, the mechanical work to compress the working fluid should be minimized and the latent heat of evaporation should also be made as small a part of the total energy used to heat the gas as possible.
Several thermodynamic cycles can be characterized as gascycle engines since the working fluid is in the gaseous or vapor form. Brayton, Otto and Diesel cycles are examples of gas-cycle engines.
The Brayton cycle consists ideally of two constant-pressure (isobaric) processes interspersed with two reversible adiabatic (isentropic) processes. Of course, no actual engine is capable of perfect isobaric or adiabatic processes since there are always irreversible losses.
The Brayton cycle is most commonly exemplified in the gas turbine engine, where compression and expansion devices handle the large volumes of working fluid. The self-contained gas turbine basically is a steady-flow device with a compressor, a combustion chamber or other heating mechanism where heat is added, and an expander element. The expander can take the form, for example, of a turbine, multiple piston or Wankel arrangement. Each of the phases of the cycle except the expander is accomplished with steady flow in its own mechanism rather than intermittently, as with the piston and cylinder mechanism of the usual Otto and Diesel cycle engines. Where the expander is a turbine, it, too, is accomplished with steady flow.
Currently, air and other gases are used as the working fluid for internal combustion cycles, including the Brayton cycle, and liquids which vaporize at appropriate engine cycle temperatures, such as water, freon, or mercury, are typically used in external combustion cycles of the Rankine cycle type. Some comparisons of the two cycles can be seen as follows: